A traditional accelerator pedal provides a mechanical linkage between a vehicle operator and a throttle valve located on the engine of a vehicle. By depressing the accelerator pedal, the throttle valve opens to allow more air into the engine while an additional quantity of fuel is also delivered to the engine. The resulting increase of fuel combustion thereby accelerates the vehicle. This type of engine fuel control is relatively low-cost, and has experienced reliable and dependable performance for a number of years.
A number of industries have been revolutionized by the age of electronics and the advent of the microprocessor, including the automotive industry. This has resulted in an ever-increasing ability to more precisely control processes and machines, especially motor vehicles. Prompted by consumer demand and government regulations, vehicle system manufacturers and suppliers are now utilizing electronics and microprocessors to perform an increasing number of tasks which were traditionally performed by mechanical apparatus. One such task is that of vehicle fuel control.
Electronic fuel control allows a microprocessor to manage the combustion process and adapt combustion to meet changing operating demands characterized by various operating modes, such as high performance, low emissions, optimal fuel economy, or constant speed control, to name a few. Early electronic fuel control systems relied upon a mechanical throttle linkage while incorporating electronic fuel injectors and electronic spark control. This was, perhaps, due to the established reliability and durability of a mechanical system compared to the relatively new electronic components. However, this type of hybrid system compromises performance features which are available to fully electronic systems. Thus, it is desirable to have a fully electronic fuel control system including an electronic accelerator pedal.
Obviously, fuel control is a very important function in the proper operation of a motor vehicle. Thus, the utilization of an electronic accelerator pedal necessitates a robust control system which is capable of fault detection and correction. Furthermore, it is desirable for the design to be fail-safe such that an unexpected component failure results in a default mode of operation or an orderly shut-down of the system.
Redundant components may be utilized where necessary to verify the proper operation of critical components or sensors. Since a throttle position sensor is one such critical component, it is desirable to provide some degree of redundancy in systems utilizing an electronic accelerator pedal.
In such systems which employ redundancy, a component fault is indicated by detecting a discrepancy between signals provided by the redundant components. It is therefore desirable to monitor signals from redundant components so as to detect any discrepancy between such signals which indicates a component fault.
A fail-safe design which incorporates redundant components is provided by an electronic accelerator pedal with an idle validation switch when used with the appropriate control algorithms. The proper functioning of a standard electronic accelerator position sensor (APS), such as one which conforms to the specifications generated by the Society of Automotive Engineers (SAE) in SAE J1843, is confirmed by an idle validation switch.
The idle validation switch is energized when the accelerator pedal is near its idle position. The idle position corresponds to the accelerator pedal being fully released. The idle validation switch, however, allows for some variation of the fully released position since it is typically energized when the accelerator pedal is depressed less than 10% of its full travel. The switch may include some hysteresis circuitry for debouncing, or this function may be performed by the vehicle controller. Since an electronic accelerator pedal is not mechanically linked to the throttle valve of the engine, the pedal position does not change with variations in engine idle speed, such as when the engine is cold compared to when the engine is hot.
The APS includes a potentiometer which is characterized by a variable electrical resistance depending upon the position of the accelerator pedal. By applying a known reference voltage across this variable resistance, a variable voltage signal is generated indicative of the accelerator pedal position. This variable voltage signal is one of the many inputs utilized by the vehicle controller to control the amount of fuel delivered to the engine. As is well known, other inputs affecting fuel control include coolant temperature, air temperature, and engine speed, among others.
Typically, the variable voltage signal is converted by the vehicle controller to a percentage of full throttle so that 0% corresponds to idle speed and 100% corresponds to full throttle. This percentage value is one parameter used to reference look-up tables which contain values representing the amount of fuel to deliver to the engine. Therefore, for consistent performance among vehicles, it is desirable to have consistent voltage values delivered by each APS.
As previously noted, the SAE has attempted to standardize the performance specifications of electronic accelerator pedals. However, the SAE J1843 specification still allows for significant variation in the acceptable ranges for an APS with an idle validation switch. Thus, a common practice in the automotive industry is to provide fault detection based on the maximum specification tolerances. This practice typically allows an error of between 5% and 15% before the controller considers a fault to exist. Due to the critical nature of fuel control, it is desirable to provide a method and system for automatically calibrating an APS equipped with an idle validation switch which conforms to that specification so as to reduce the error tolerance before a fault is indicated.